Figure 2.

Novel RNA-mediated regulations of L. monocytogenes gene expression. (A) Control of AsPocR by a B₁₂ riboswitch, adapted from Cossart and Lebreton.¹⁰³ (Left) In absence of B₁₂, full-length AspocR is produced and inhibits pocR expression. (Right) In presence of B₁₂, the transcription of AsPocR is terminated prematurely by the B₁₂ riboswitch (RS), and transcription of pocR is allowed, provided that propanediol (PDO) is also present in the medium. (B) Sequestration of an antiterminator by a B₁₂ riboswitch, inspired from Mellin et al.⁷⁵ In presence of Ethanolamine (Ea), the antiterminator EutV is phosphorylated by EutW and activated. (Left) In absence of B₁₂, full-length Rli55 is produced and sequesters active EutV. The transcription of eut genes is prevented by early termination at the ANTAR elements. (Right) In presence of B₁₂, the transcription of Rli55 is terminated prematurely by the B₁₂ riboswitch. Free phosphorylated EutV dimers can then bind the ANTAR element and mediate antitermination, allowing the expression of eut genes. (C) The mogR-fli locus excludon, adapted from Cossart and Lebreton.¹⁰³ mogR is encompassed by 2 transcripts. In the longest one, the 5′-UTR of upstream of MogR coding sequence also acts as an antisense RNA, Anti0677, overlapping the fli operon. The two divergent transcriptional units encode proteins of opposite functions: MogR is a transcriptional repressor of flagellum genes, whereas Fli proteins participate in the flagellum export apparatus. (D) Rli27 allows the intracellular-specific translation of an alternative transcript, inspired from Quereda et al.⁹⁴ Extracellular bacteria express mainly lmo0514-lmo0515 from the constitutive P1 promoter. An alternative, long mRNA is produced in low amounts from P2, but ribosomes cannot access the Shine-Dalgarno sequence (SD). In cells, the P2 promoter is activated, and Rli27 is expressed. Base-pairing of Rli27 with the 5′-UTR of the long transcript allows ribosome binding and translation to proceed.